An orthodontic coil spring made of an alloy wire exhibiting shape memory properties, such as nickel-titanium (Ni-Ti) alloy wire, is shown in U.S. Pat. No. 4,849,032. The Ni-Ti alloy wires exhibiting shape-memory properties are often referred to as "shape-memory alloy wires." Shape-memory alloy wires frequently exhibit excellent superelastic and spring-back properties.
Superelasticity occurs when the stress value remains substantially constant up to a certain point of wire deformation, and when the wire deformation is reduced, the stress value again remains substantially constant. Therefore, a coil spring made of a shape-memory alloy wire can maintain a substantially constant load value throughout a zone of deflection. Because shape-memory alloy wires possess excellent spring-back properties, they can also be deflected to greater degrees than other types of wires, without causing permanent deformation of the wire.
A shape-memory coil spring is made by winding a shape-memory alloy wire, such as a Ni-Ti alloy wire, into a coil. If the coil spring is to be used as an open or compression coil spring, then it is wound into a coil defining spaces between each turn thereof. If the coil spring is to be used as a closed or tension coil spring, then it is tightly wound into a close contact shape substantially without any spacing between its turns. Tension coil springs are provided with hooked portions on their ends to connect the springs to orthodontic appliances.
In FIG. 1, a typical open shape-memory alloy coil spring is indicated by the reference numeral 1. The coil spring 1 is fitted over an archwire 2 and mounted between two orthodontic brackets 3. The orthodontic brackets 3 are in turn mounted to adjacent teeth T1 and T2. The coil spring 1 is an open or compression coil spring, and is thus wound with spacing between its turns. As indicated by the arrow shown in FIG. 1, the coil spring 1 is used to shift the tooth T1 away from the tooth T2, and thus into the space between the teeth T1 and T3.
One advantage of the coil spring 1 is that because it is made of a shape-memory alloy wire, it exerts a substantially constant spring force throughout a zone of deflection, often referred to as the "superelastic zone of deflection." Therefore, if the distance that the tooth T1 is to be shifted is within the superelastic zone of deflection of the spring 1, the spring 1 can be used to apply a substantially constant spring force throughout the entire movement of the tooth.
An open shape-memory alloy coil spring, like the coil spring 1, is typically made by winding a shape-memory alloy wire into a continuous-length open coil spring. The continuous-length coil spring is wound to define substantially constant spacing between the turns thereof. The continuous-length coil spring is heat treated, and is then cut into a number of shorter-length coil springs. The length of each shorter coil spring is dimensioned so that the spring will be compressed when fitted over an archwire in the space provided between the orthodontic brackets mounted on a patient's teeth.
One problem with open shape-memory alloy coil springs, is that because they are cut from larger continuous-length coil springs, the ends of each coil spring are not adapted to properly engage the orthodontic brackets mounted to a patient's teeth. Usually, the ends of each spring are cut at the middle of a turn or, that is, at the midpoint of the space between two turns of the spring. As a result, the free ends of such a coil spring are oriented at oblique angles relative to the longitudinal axis of the spring. Thus, when the spring is mounted over an archwire, only the tips of its free ends engage the orthodontic brackets.
The tips of the spring, however, are not shaped or oriented to conformably engage the surfaces of the brackets. This problem is enhanced with open shape-memory alloy coil springs, because they are typically deflected or opened to a greater degree than other types of orthodontic open coil springs. As a result, the force of an open shape-memory alloy coil spring is usually not applied to the orthodontic brackets in a smooth and reliable manner.
It is an object of the present invention, therefore, to provide an orthodontic coil spring that overcomes the problems of known shape-memory alloy coil springs.